District Judge William Morvant said the amendment was flawed as drawn up by the Legislature because it had more than one purpose: banning not only gay marriage but also civil unions.

Anti-gay marriage groups expressed outrage, and attorneys vowed to appeal.

"We have judges acting in arrogance to usurp the actions of the Legislature and deny the voters of Louisiana who voted overwhelmingly to support the protection of marriage," said Tony Perkins, president of the Family Research Council in Washington.

A gay-rights group called Forum for Equality had challenged the amendment, arguing among other things that combining the question of gay marriage and the issue of civil unions in one ballot question violated state law.

"Somebody got greedy and wanted to stick something else in. They got caught with their hand in the cookie jar," said Forum for Equality attorney John Rawls, calling the ruling "unassailable, irrefutable, plain and clear."

The courts had rejected a similar argument before the Sept. 18 election, saying it was premature.

Some 78 percent of voters favored the amendment. The vote was part of a national backlash against gay marriage, which followed last year's Massachusetts ruling allowing gay couples to wed.

Proposals to restrict marriage to a man and a woman are on the November ballot in 11 states: Arkansas, Georgia, Kentucky, Michigan, Mississippi, Montana, North Dakota, Ohio, Oklahoma, Oregon and Utah. Missouri voters overwhelmingly approved such an amendment earlier this year.

The Louisiana Legislature put the proposed ban on the ballot this spring. Louisiana already had a law against gay marriage, but conservatives warned that unless it was put in the state constitution, a Louisiana court could one day follow the Massachusetts example.

The judge appeared to have been influenced by the testimony of a New Orleans man, William Schultz, who told the court Tuesday that voting for the ban posed a dilemma for him, because he opposed gay marriage, but favored civil unions.

The judge's ruling also was preceded by an emotional appeal from Rawls, who pleaded with the judge to uphold the rights of the "most despised minority."

But the judge, an elected Republican and veteran of this city's drug court with a reputation for no-nonsense efficiency, rejected such appeals to "emotions," saying firmly: "This is a matter of law. Emotions do not, will not play a part in this court's ruling."